This invention relates to high current discharge circuits that utilize a silicon-controlled rectifier (SCR) and, more particularly, to such circuits in which there is improved control over operation of the SCR. The invention is particularly, but not exclusively, useful in a sonic graphical digitizer.
Graphical digitizers are conventionally used to input graphical coordinate information, or the like, to a companion system. In a graphical digitizer, wave energy is typically passed between a movable element (such as a stylus or cursor) and one or more transducers located at fixed reference locations. The transit time of the wave energy traveling (in either direction) between the movable element and the reference locations is used in determining the position of the movable element in one, two, or three dimensions, typically in terms of digital coordinates. A type of graphical digitizer manufactured and sold by the assignee hereof, Science Accessories Corporation, measures the transit time of acoustic or sonic energy propagating through air. The transit time of sound traveling from a source on the movable element to each of a plurality of microphones is used, in conjunction with the velocity of sound in air and known geometrical relationships, to compute the position of the movable element.
In a sonic digitizer of the type described, a fast rise time pulse of acoustic energy is typically generated by applying a high voltage pulse to electrodes of a transducer, such as spark gap or a piezoelectric ceramic between capacitive plates. There is a high current discharge at the transducer which produces the desired sonic pulse. The type of circuit used to obtain the sonic pulse is illustrated in FIG. 1. A supply voltage, V.sub.s, is utilized to charge a capacitor C.sub.1 via a resistor R.sub.1. The capacitor has a discharge path through the primary winding of transformer T.sub.1 and a silicon controlled rectifier (labeled SCR), when the SCR is conductive. As is known in the art, trigger pulses are applied, at appropriate times, to the trigger gate electrode g of the SCR to render the SCR conductive and cause a pulse of relatively high voltage across the transformer secondary winding. A transducer such as a spark gap 125 is coupled across the secondary winding, and a quick discharge occurs at the transducer. When the SCR turns off, the capacitor can again be charged and awaits the next trigger pulse. The trigger pulse is also typically used to start the digitizer counters. [In my copending U.S. patent application Ser. No. 495,361, filed Mar. 16, 1990, and assigned to the same assignee as the present application, there is disclosed a technique for starting the counters in response to detection of the spark generation at the transducer.]
The type of circuit illustrated in FIG. 1 suffers from a fundamental problem; viz., the SCR, once turned on, cannot be turned off under the control of the gate lead. Instead, the anode-cathode current must be reduced below some minimum value, known as the "minimum holding current", for some period of time until its internal structure is cleared of stored charge and it turns off. Positive anode voltage can then once again be applied. For further information on this phenomenon, reference can be made to Section 1-5 of Motorola Catalog DL137, entitled "Thyristor Device Data".
In one prior approach to this problem, SCR turn-off was achieved by making EQU V.sub.DC /R.sub.C &lt;I.sub.H
where I.sub.H is the holding current and can be empirically determined. This technique has the disadvantage that I.sub.H is so low that large values of R.sub.C are required. The time required to charge C.sub.D is consequently long, which limits the pulse generation rate.
Another proposed method of obtaining SCR turn-off was to replace the DC source voltage with a rectified sine wave from a 60 HZ power transformer. In this case, the supply voltage falls to zero every 8.33 ms. permitting operation with a lower value of R.sub.c. This technique has a number of disadvantages, including: 1) low pass filtering cannot be interposed between the supply voltage and R.sub.c, and transient primary current will be coupled back into the power transformer; 2) since the triggering rate and the line are asynchronous, charging time can vary, and at high pulsing rates, acoustic output can vary substantially from pulse to pulse; 3) no regulator can be interposed, and the acoustic output will vary with line and load variations of the power transformer.
A still further proposed approach to SCR turn-off makes use of a clamp diode at the anode of the SCR to convert the current flow in the transformer primary at the end of a pulse to a reverse current for a long enough period to turn off the SCR. This approach also has disadvantages. One such disadvantage is that it imposes some undesirable limitations on the component values that can be used in filtering the transformer secondary output. Another disadvantage is that if the electrodes of the secondary circuit are short circuited, the SCR can fail to turn off, which can burn out the primary circuit.
It is among the objects of the present invention to provide solution to the problems and limitations of the prior art as set forth.